The present invention relates to thermal printers, and, in particular, to thermal printers for printing quarter-tone images on the reverse side of image-bearing media.
In certain printing equipment, such as silver-halide printers, or electro-photographic printers, or thermal printers, large numbers of images can be produced over time. Especially in large volume printing applications, it becomes important to identify individual prints with such information as the owner of the print or the condition that existed to create the print or the source of the image. In these printers, such information is printed on the back, or non-image bearing, reverse surface of the prints. This is especially true in photographic style images, that cover the entire obverse surface of the print. In these photographic style prints, it is undesirable to have the labelling information on the front surface or attached as a tag.
In photographic systems, several techniques have been used to create labels on the back of the prints. In the Kodak 3510 printer, for instance, an impact printing system is used to record information on the back of prints. These impact printers typically have a ribbon that carries dye disposed in a vegetable oil. The impact of the hammers transfers some of the ink to the back of the print. One disadvantage of this method is that the ink can smear in the presence of water and solvents. Another disadvantage of this method is that high impact forces can cause marks on the silver halide emulsion.
Recently, improvements have been made in thermal printing systems that have made the use of thermal printers attractive in marking the backs of prints. See, for example, the thermal backprinting apparatus shown in U.S. Pat. No. 4,629,312. This technology uses a thermal print head, which consists of a number of thermally resistive elements, to transfer compounds from a carrier web. In one embodiment, the marking compound is a wax compound. These compounds tend to smear and have poor durability. More recently, compounds have been formulated of resins such as polyethylene or polyolefins. These polymers have exhibited improved wear, solvent and scratch resistance.
Using a thermal marking system to label the backs of prints from thermal, electrophotographic and silver-halide printers presents several problems. The thickness of the media receiving backprinting varies from 0.005 to 0.007 inches. The image material is a compound structure of paper with two sided coatings of a low density polymer, such as polypropylene or poly-ethylene. As such, the media are generally translucent. The thermal transfer material is a resin laden with carbon black. A typical thermal printer uses a print head with a linear array of 512 heating elements to create a 7.times.9 pixel array for each character. The transfer material is engaged by the head, pressed against the back of a print, and the array is energized. After backprinting, the dark, backprinted text forms a shadow on the obverse image. Such a shadow is especially noticeable when the print was placed against a white background or held up to bright lights. Such shadows are highly objectionable and render thermal backprinting unsuitable for commercial use.